Fiber reinforced composite materials in which carbon fibers or aramid fibers are used as reinforcing fibers have high specific strength and high specific modulus and are thereby widely used as structural materials for aircrafts and automobiles and for general industrial and sports applications such as tennis rackets, golf shafts, and fishing rods.
In one method of producing a fiber reinforced composite material, sheets of prepreg, which is an intermediate sheet-shaped material prepared by impregnating reinforcing fibers with an uncured matrix resin, are stacked and then heat-cured. Also, a resin-transfer-molding method is used in which a liquid resin is poured into reinforcing fibers placed in a mold and is then heat-cured.
Among these production methods, the method that uses a prepreg has an advantage in that a high-performance fiber reinforced composite material is easily obtained because the orientation of the reinforcing fibers can be strictly controlled and the design flexibility of the laminated structure is high. From the viewpoints of thermal resistance and productivity, a thermosetting resin is mainly used as the matrix resin for the prepreg. An epoxy resin is particularly preferably used because of its mechanical properties such as adhesive properties to the reinforcing fibers.
The epoxy resin has higher elastic modulus but lower toughness than a thermoplastic resin, and therefore the impact resistance of the fiber reinforced composite material is insufficient.
To improve the toughness of an epoxy resin, various attempts have been made; for example, a rubber component or thermoplastic resin having high toughness is added to form a structure including a phase separated from the epoxy resin. However, those methods have problems such as a reduction in elastic modulus, deterioration of thermal resistance, deterioration of processibility due to an increase in viscosity, and deterioration of quality such as occurrence of voids. For example, Patent document 1 proposes a method of greatly improving the toughness of an epoxy resin by adding a styrene-butadiene-methyl methacrylate copolymer or a block copolymer such as a block copolymer of butadiene-methyl methacrylate to stabilize the formation of a fine phase-separated structure during the curing process of the epoxy resin. Patent document 2 discloses that the addition of a large amount of a thermoplastic resin, such as a phenoxy resin or polyethersulfone, which forms a separate phase allows a significant improvement in toughness and a significant increase in minimum viscosity; therefore, when the resultant mixture is used as a self-adhesive prepreg for the face plate of a honeycomb panel, sufficient fillets are formed on the bonding surface with the honeycomb core, and high self-adhesive properties are thereby obtained. However, it is inferior in universal use: For example, when using a prepreg for an aircraft primary structural material required to have higher mechanical properties and longer reliability, voids and irregular orientation of fibers are likely to occur in a molded product due to increase in viscosity caused by the addition of the thermoplastic resin, thus not demonstrating enough performance.
A combination of various epoxy resins can compensate for their drawbacks and can provide better-balanced properties than a single component resin. However, with such a combination, a phase-separated structure is generally not formed, and therefore the toughness is improved only slightly, so that the impact resistance of the fiber reinforced composite material is hardly improved. For example, when an amine-type epoxy resin having high elastic modulus is added to a bisphenol-A-type epoxy resin, which is an epoxy resin having high toughness, the resultant toughness and elastic modulus take values between these of the components, and there is the problem in that the impact resistance is not improved.
For example, in Patent documents 3 and 4, an amine-type epoxy resin having high elastic modulus is added to a bisphenol-type epoxy resin, thus significantly improving bending strength in fiber direction and interlayer shear strength that are strongly correlated with compression strength in fiber direction. However, the resin toughness and impact resistance are not sufficiently improved.